In recent years, with the rise in the number of pixels of the image pickup element, demand for an optical system having a lower error sensitivity, which cannot be easily influenced by manufacturing errors, while maintaining high optical performance with high design flexibility has been growing. Technologies such as a zoom optical system including three lens groups have been proposed in response to the demand. The three lens group zoom optical system comprises lenses having a negative power, positive power and positive power (which is also called refractive power defined by an inverse number of a focal length) in order from an object side.
Further, from the portability and the design points of view, an optical system by which a thin shaped camera body can be designed has been demanded. In response to this demand, proposed conventionally has been a zoom optical system including a first lens having negative power in which a reflecting surface for inflecting an optical path is disposed on the optical path of the zoom optical system.
Further, an optical system has been proposed in which a short length optical system is realized by forming a telephotometry optical system structured by a lens group in which negative power, positive power and negative power lenses are disposed in order from an object side, while making a camera thin by disposing a reflective surface in the first lens group.
Moreover, a zoom optical system structured by lens groups in which negative power, positive power, negative power and positive lenses are disposed in order from an object side where the first and fourth lens groups are fixed has been proposed. The zoom optical system that has four lens groups is suitable for minimizing the size of a zoom optical system.
However, since a focusing operation is executed by the third lens group in the optical system that includes three lens groups, the third lens group moves toward the object side and the distance between the second lens group and the third lens group becomes narrow when the object distance becomes short. Consequently, there is a problem in that it is difficult to make a shortest object distance at a telephoto point in which a moving distance is particularly long.
Further, in order to shorten the closest object distance, it is necessary to set the distance between the second lens group and the third lens group longer when setting an infinite object distance. In this case, as the third lens group approaches the image surface side, it becomes difficult to utilize the third lens group to adequately correct aberration. Further, there is a problem in that the size of the total lens system becomes large when securing the adequate distance between the third lens group and the image surface in order to avoid the problem described above.
In the optical system wherein the camera thickness is made thin by disposing the reflective surface in the first lens group, moving a negative third lens group toward the image surface side makes it possible to realize focusing at close range and to have a structure which can easily shorten the closest object distance. However, there is a problem that since the concave surface of the first surface of the lens located in the most object side is arranged to face to the object side, off-axis aberration occurs and many aspherical surfaces are required to correct the off-axis aberration. Further, there is a problem that the total length of the lens has to be prolonged.
In the optical system including four lens groups, when securing the necessary distance for providing a reflective optical element between the first and second lenses, since the first lens is configured by a negative single lens, off-axis aberration occurs like a case described above. The problem is that in order to correct the off-axis aberration, many aspherical surfaces are required and the total length of the lens has to be prolonged.